Coal cleaning process

ABSTRACT

Fine particle coal is beneficiated in specially designed dense medium cyclones to improve particle acceleration and enhance separation efficiency. Raw coal feed is first sized to remove fine coal particles. The coarse fraction is then separated into clean coal, middlings, and refuse. Middlings are comminuted for beneficiation with the fine fraction. The fine fraction is deslimed in a countercurrent cyclone circuit and then separated as multiple fractions of different size specifications in dense medium cyclones. The dense medium contains ultra-fine magnetite particles of a narrow size distribution which aid separation and improves magnetite recovery. Magnetite is recovered from each separated fraction independently, with non-magnetic effluent water from one fraction diluting feed to a smaller-size fraction, and improving both overall coal and magnetite recovery. Magnetite recovery is in specially designed recovery units, based on particle size, with final separation in a rougher-cleaner-scavenger circuit of magnetic drum separators incorporating a high strength rare earth magnet.

This is a divisional of application Ser. No. 07/775,860, filed Oct. 15,1991, which is a continuation-in-part of application Ser. No. 07/492,312filed Mar. 6, 1990, now U.S. Pat. No. 5,096,066 issued Mar. 17, 1992,which is a continuation of application Ser. No. 07/126,419 filed Nov.30, 1987, now abandoned

FIELD OF THE INVENTION

The present invention is directed generally to the field of coalcleaning processes, and in particular, is directed to removal of refuse,such as sulfur-containing minerals, from fine coal particles.

BACKGROUND OF THE INVENTION

Coal is a widely used, but limited, fuel for generating electricity inthe United States and around the world. However when burned, coal canemit significant amounts of pollutants which create environmentalproblems. Environmental concern is exemplified by the Clean Air ActAmendments of 1990 creating new emissions limitations for coal of 2.5pounds of sulfur dioxide per million BTU effective in 1995 and 1.2pounds of sulfur dioxide per million BTU effective in the year 2000.

A utility that burns high sulfur coal currently has the option ofswitching to a low sulfur coal or scrubbing flue gases to remove sulfurdioxide. Scrubbing sulfur dioxide requires significant capitalinvestment and is operationally expensive. For many utilities, switchingto low sulfur coal would be very expensive due to transportation costsfor delivering coal from a distant source and capital costs associatedwith plant modification to accommodate coals with different combustioncharacteristics. Substantial deposits of high sulfur coal currently fuelmany electrical power generation plants. A need exists to improve thecleaning of sulfur from such coals prior to combustion so that they maybe efficiently used without producing excessive pollutants.

Beneficiation of coal refers to the removal of non-coal material fromraw coal to produce a relatively clean coal product. Raw coal iscomposed of high purity coal material and non-coal material. Non-coalmaterial in coal, commonly referred to as ash, normally includes pyrite,clays, and other aluminosilicate materials. The presence of largeamounts of these ash materials can create problems during combustion,such as slagging and fouling. Sulfur is present in raw coal in twoforms, organic sulfur and inorganic sulfur. Organic sulfur is chemicallybound as part of the coal matrix. Inorganic sulfur is all sulfur notchemically bound in the coal matrix. Pyrite sulfur is the predominateform of inorganic sulfur. Sulfate sulfur is another form of inorganicsulfur associated with ash forming materials. Physical beneficiationeffectively removes only inorganic sulfur. Processes for beneficiatingcoal are varied, but commonly utilize dense medium separation, jigs, orfroth flotation to separate clean coal from non-coal material. Becauseof its versatility, high efficiency and ease of operation, dense mediumseparation is perhaps the preferred separation technique.

In dense medium separation, raw coal is introduced into a medium havinga specific gravity intermediate between that of coal and non-coalmaterial. The dense medium may be a homogeneous liquid, but is moreoften composed of water and magnetic particles, such as ferromagneticparticles. Magnetite is a commonly used magnetic particle. Separationcan be carried out in a dense media bath or tank, or in a cyclone. Whena cyclone is used, coal is generally removed as the overflow productwhile refuse becomes the underflow product. After separation of coal andrefuse, it is advantageous to recover the magnetic particles from thecoal and from the refuse for reuse.

Raw coal feed, typically known as run-of-mine coal, is a mixture ofthree components, namely organic material, rock and pyrite. In raw coal,some particles are liberated, meaning that they constitute relativelypure components. Other particles are locked, meaning that theseparticles contain two or more of the three components locked together.Such locked particles are referred to as middlings.

Each of the raw coal components has a characteristic specific gravity.To illustrate, organic material has a specific gravity of about 1.25,rock has a specific gravity of about 2.85 and pyrite has a specificgravity of about 5.0. A raw coal feed contains particles with manyspecific gravities as a result of the differing specific gravities ofthe three separate components and the combination of components whichare locked together.

While dense medium beneficiation has been effective for large size coalfeed particles, those greater than approximately 0.5 mm in size, it hastypically not been used for smaller-size coal particles. In this regard,the separation efficiency for small particle coal feeds has not beensatisfactory. As a result, small coal particles are often discarded.

One way to improve the separation of coal from non-coal material is tocrush or otherwise comminute the raw feed to liberate high purity coaland non-coal material in the middlings. Generally, as the average sizeof the particles in the raw coal feed becomes smaller, more coal andnon-coal material are liberated and the percentage of particlesconstituting middling decreases, potentially allowing the recovery ofmore coal product. Crushing or grinding a coal feed to liberate coallocked with non-coal material in middlings has not been practicalbecause there was no process for treating fines which efficientlyseparates coal from non-coal material. Middlings material, therefore,either reports to the clean coal, which introduces pyrite and otherunwanted minerals into the fuel, or reports to the refuse resulting inan undesirable loss of coal. Comminution of an entire coal feed is,however, costly and not commercially practical. The expense ofcomminution is significant and it would be desirable to minimize thecosts.

As indicated above, in order to recover coal from middlings to produce ahigh purity coal product, it is necessary to comminute the middlings andthen to separate the coal from refuse. If middlings are not processedfor further coal recovery, a substantial quantity of useable coal in themiddlings will be discarded along with non-coal material. Accordingly,to maximize recovery of a clean coal product, it is essential to developbeneficiation processes designed to handle small particle raw coal feed.

U.S. Pat. No. 4,364,822, by Rich, issued Dec. 21, 1982, describes a coalcleaning process involving two-stage cyclone separation that producesthree products, clean coal, refuse, and middlings. Middlings are thencrushed and recycled through the cyclones with the raw coal feed. Rich,however, specifically teaches away from a dense medium process usingmagnetic particles based on problems with the recovery of magneticparticles .

U.S. Pat. No. 3,908,912 by Irons, issued Sep. 30, 1975, describes aprocess whereby refuse is initially separated out at high density,followed by a lower density separation to yield clean coal andmiddlings. Middlings are then crushed for further cleaning. However, inIrons small size coal is not removed from the coal feed prior to theinitial high density separation which results in additional refuse inthe clean coal product. Moreover, Irons discloses that cycloneseparations of small coal fines are inefficient in that particles arefrequently misplaced. As such, Irons teaches the use of secondarycyclones followed by flotation to eliminate refuse in the coal.

Many attempts have been made to clean fine particle coal, with varyingresults. In dense medium cycloning, separation efficiency drops as coalfeed particles become smaller. In particular, considerable difficulty isencountered in cleaning a coal feed made up of particles less than about0.5 mm in size. Also, recovery of the magnetic particles from the densemedium after beneficiation becomes more difficult as coal feed particlesbecome smaller.

Accordingly, there is a need for an effective and efficient means forbeneficiating coal feed particles less than about 0.5 mm in size wherethe separation efficiency is sufficient such that the coal product meetsdesired specifications. The separation efficiency of a coal cleaningprocess is frequently illustrated through probability curves known aspartition curves. These curves describe the probability that a givenparticle in the feed will report to the clean coal rather than refuse.The measure of the slope of the vertical portion of a partition curve isthe separation's probable error, or Ep. The more vertical the centerportion of the partition curve, the more efficient the separation andthe smaller the probable error.

In order to avoid the difficulties associated with cleaning small sizeparticles, many methods for processing fine coal particles discardparticles below a threshold size prior to beneficiation, typicallyreferred to as desliming. Desliming has traditionally been based onlimitations of the beneficiation process. For example, U.S. Pat. No.3,794,162 by Miller et al., issued Feb. 26, 1974, discloses a heavymedium beneficiating process for particles down to 150 mesh (0.105 mm).Particles smaller than 150 mesh are screened-out prior to beneficiationby dense medium cyclone. U.S. Pat. No. 4,282,088 by Ennis, issued Aug.4, 1981, discloses a process where particles smaller than 0.1 mm areseparated out in a cyclone classifier and discarded prior tobeneficiation by dense medium cyclone. When all particles below 0.1 mmor 0.105 mm in size are discarded, pure coal is also discarded both assmall coal particles and as coal locked in small middling particles.

The ability to deslime by screening or sieving is limited by availablescreen and sieve construction. Screening or sieving large quantities ofmaterial below a size of about 150 mesh is not practical. Classifyingcyclones, which separate particles based on different particle settlingvelocities, have been used to classify coal feeds, but have not beeneffective for making a size classification of coal feed at 0.015 mm.Rejecting only the smallest coal particles in raw coal feed, on theorder of 0.015 mm and smaller, presents a major problem. Particlessmaller than this size are predominately refuse material which should bediscarded.

One parameter in cyclone design which has received relatively littleattention is the size of the inlet orifice through which feed enters thecyclone. Arterburn, in a paper entitled, "The Sizing of Hydrocyclones"(Krebbs Engineers 1976), notes that feed orifices usually have an areabetween 6 percent and 8 percent of the area of the cyclone feed chamber.The modification of inlet diameters has not been identified as a factorto improve a classifying cyclone separation capability.

Multiple classifying cyclones arranged in a countercurrent flow circuithave been used for size classification of starch. U.S. Pat. No.4,282,232 by Best, issued Aug. 11, 1981, describes a countercurrentcyclone circuit designed primarily to wash starch. As far as theinventor knows, a countercurrent arrangement of classifying cyclones isnot practiced in the coal cleaning industry and has not been used toseparate particles of the magnitude of 0.015 mm and smaller.

Attempts have been made in the coal industry to eliminate the need fordesliming by improving the beneficiation process. For example, U.S. Pat.No. 4,802,976, by Miller, issued Feb. 7, 1989, discloses a process inwhich froth flotation is used to recover coal particles smaller than 28mesh (0.595 mm) downstream of a dense media cyclone. But this process isnot appropriate for all coals. A raw coal feed often contains oxidizedcoals which do not float. Also, pyrite tends to float, along with cleancoal, thereby contaminating the clean coal product. Devising a processto treat all types of fine particle coal and to effectively removepyrite from the smallest size fractions, has been problematic.

Cyclones for use in connection with dense medium beneficiation havevarying size parameters and can be subject to varying operatingconditions. In general, cyclones do not operate as effectively when usedto beneficiate small size particles. A problem with the use of cyclonesfor the beneficiation of small coal particles is the need to assure thatthe particles correctly report to either the underflow as refuse oroverflow as coal. Small particles often become misplaced, therebydecreasing the separation efficiency of the cyclone.

One cyclone parameter is the area of the inlet orifice through which rawcoal feed enters the cyclone. U.S. Pat. No. 2,819,795 by Fontein, issuedJan. 14, 1958, discloses a cyclone design where the area of the inlet iscalculated to equal between 0.1 and 0.4 times the area available foroverflow. Fontein also specifies a cyclone diameter between two andthree times the diameter of the overflow. Fontein does not discuss theinlet diameter as related to the cyclone diameter or particle velocity.U.S. Pat. No. 4,341,382 by Liller, issued Jul. 27, 1982, discloses adesign for an eighteen inch diameter cyclone where the inlet tubediameter is calculated to equal between 0.25 and 0.35 times the cyclonediameter.

Fourie et al., "The Beneficiation of Fine Coal by Dense-MediumCyclones", Journal of South African Institute of Mining and Metallurgy,pp. 357-361 (October 1980), discloses the use of magnetite particles inbeneficiating minus 0.5 mm coal by dense medium cycloning where at least50 percent of the magnetite is finer than 10 microns (0.010 mm). Finersize magnetite is, however, more difficult and costly to recover fromclean coal and refuse. Fourie discloses the recovery of magnetite in arougher-cleaner-scavenger arrangement of wet drum magnetic separatorsand reported serious problems with magnetite loss. There is a need for aprocess which employs magnetite small enough to separate fine size coaland refuse effectively, but allows for sufficient recovery of magnetiteafter beneficiation.

Magnetite used in Fourie was prepared by milling magnetite ore. Butmilling ore to ultra-fine sizes is very expensive, and milling giveslittle control over particle size distribution. Magnetite for use indense media separation can also be produced by chemical reduction ofhematite. U.S. Pat. No. 4,436,681 by Barczak, issued Mar. 13, 1984,discloses a process whereby hematite prepared by spray roasting of ironchloride is reduced to magnetite. However, Barczak does not discussmagnetite particle size or recognize problems encountered duringmagnetite recovery following dense medium separation.

U.S. Pat. No. 4,777,031 by Senecal, issued Oct. 11, 1988, discloses aprocess whereby magnetite is produced by pyrohydrolysis of iron chlorideat temperatures between 1000° C. and 1600° C. However, Senecal isdirected to producing magnetite particles between 0.02 and 0.2 microns(0.00002 mm to 0.0002 mm) in size that are well suited for bindersystems such as those used in magnetic recording media. Senecal'sprocess results in magnetite particles too small to be used effectivelyin dense medium separation of coal due to problems with recovering suchsmall particles following dense medium separation.

Magnetite used in dense medium separation has traditionally beenrecovered for reuse by first draining the medium from the separatedproduct over screens and then rinsing the product over screens to removethe remaining magnetite. Magnetite is then separated from the rinsewater, dilute medium, by magnetic separation. However, when cleaningfine size coal particles, screens are not effective to keep coal andrefuse particles from passing through with the medium and rinse water.These fine particles of coal and non-coal material contaminate the densemedium and are difficult to separate from the magnetite in conventionalmagnetic drum separators.

Another problem with the recovery of small magnetite particles is thatit is difficult to separate the magnetite from rinse water by magneticseparation. U.S. Pat. No. 4,802,976 by Miller, issued Feb. 7, 1989,proposes recovering magnetite as the sink from froth flotation cells,thereby avoiding the problem of fine coal and non-coal particlesentrained with magnetite during magnetic separation. Froth flotationsystems are, however, complex and difficult to operate. The use of amagnetic separator incorporating a high density gradient magnet in amatrix design could be employed. However, high density gradient magnetsare expensive and matrix separators complicate operation compared totraditional magnetic drum separators. There is a need for an effectiveseparation process using easier to operate magnetic separators and moreeconomical designs for magnetic separation.

In order to satisfy utility combustion requirements, the clean coalproduct from beneficiation must be dewatered to reduce its moisturecontent. Fine particle coal is more difficult to dewater thanlarger-size coal because of its greater surface area.

In light of the foregoing, what is needed is an improved process forbeneficiating fine particulate coal such that desired specifications,such as sulfur content, can be met. Many of the problems impedingdevelopment of such a process have been described, and they areformidable. A need exists for a process that maximizes the recovery ofcoal without the expense of comminuting the entire coal feed. Also,methods of classifying coal particles based on size must be improved,particularly methods using classifying cyclones. Improved separationefficiency of fine particle coal in high throughput dense mediumcyclones is desired. Methods to effectively recover ultra-fine sizemagnetic particles for reuse following dense medium separation areneeded to improve the viability of dense medium separation of fineparticle coal. Improved methods are also needed for producing magneticparticles of optimum size to effect good dense medium separation whilemaximizing magnetic particle recovery.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a processfor beneficiating fine particle coal in specially designed dense mediumcyclones to improve particle acceleration and to enhance separationefficiency is provided. Raw coal feed is first sized to isolate thecoarse and fine coal fractions. The coarse fraction is then separatedinto clean coal, middlings, and refuse. Middlings are comminuted forbeneficiation with the fine fraction. The fine fraction is deslimed in acountercurrent classifying cyclone circuit and then separated intomultiple fractions according to size prior to dense medium cycloning.

The dense medium contains ultra-fine magnetic particles of a narrow sizedistribution that aids separation and improves subsequent recovery ofthe magnetic particles. Magnetic particles are recovered from the cleancoal and refuse fractions independently. Magnetic particle recovery isin specially designed recovery units, based on particle size, with themore conventional drain-and-rinse approach applied to coarser fractions,but with final separation in a rougher-cleaner-scavenger circuit of wetdrum magnetic separators incorporating a high strength rare earthmagnet. The overall coal processing circuit can be arranged so that thenon-magnetic effluents from the magnetite recovery systems of coarsercoal fractions, which may contain some unrecovered fine magnetite,ultimately flow to the rougher-cleaner-scavenger circuit which recoversvirtually all fine magnetite.

One advantage of the present invention is that it constitutes aneffective process for beneficiating coal particles smaller then 0.5 mm.An advantage of one embodiment of the invention is that it provides aprocess for desliming of raw feed coal prior to beneficiation whichminimizes the amount of coal discarded as slimes and aids in subsequentmagnetic particle recovery and dewatering of the coal product.

In accordance with an embodiment of the present invention, a process isprovided for classifying ultra-fine particles employing a classifyingcyclone having an inlet area within a specified range. In anotherembodiment of the invention, a process is provided for classifying smallparticles by size using multiple classifying cyclones. While referenceto the partitioning of particles by classification cyclones to overflowand underflow is described as being by size, it is recognized thatclassification is by settling velocity which is influenced not only bysize but also by other particle parameters including particle specificgravity and shape. Another embodiment provides a process for recoveringmagnetic particles used in dense medium cycloning involving sizing andclassifying the coal feed into narrow size fractions for processing.

In accordance with one embodiment of the invention, a process forbeneficiating extremely fine coal by dense medium separation usingmagnetic particles of a particular particle size and size distributionis provided. In accordance with another embodiment, magnetite isproduced by reduction of hematite, which magnetite exhibits propertiesdesirable for dense medium separation and for which recovery is improvedfollowing separation. In accordance with another embodiment, a processof dense medium separation of extremely fine particle coal in a cyclonewith inlet area sized within a specific range is provided.

In accordance with another embodiment of the present invention, aprocess for recovering magnetic particles following dense mediumseparation is provided whereby the non-magnetic effluent from themagnetite recovery unit of a larger-particle-size coal fractioncontaining both unrecovered clean coal and magnetite is sent to circuitstreating smaller-size coal fractions, which circuits employ arougher-cleaner-scavenger magnetite recovery circuit which capturesvirtually all the magnetite while also recovering the coal. Inaccordance with another embodiment of the present invention, a processof wet drum magnetic separation using a rare earth magnet is provided.In accordance with another embodiment, a process for dewatering andagglomerating extremely fine coal involving the addition of paper fibersto the coal is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and 1B are flow diagrams of an embodiment of the process of thepresent invention.

FIG. 2 is a flow diagram depicting the high density and low densityseparations of coarse coal yielding three products.

FIG. 3 is a flow diagram depicting the sizing of a coal feed into threefractions based on size.

FIG. 4 is a flow chart schematically depicting the magnetite recoverycircuit for a larger-particle-size fraction of minus 0.5 mm coal feedfollowing dense medium separation.

FIG. 5 is a flow chart schematically depicting the magnetite recoverycircuit for a smaller-particle-size fraction of minus 0.5 mm coal feedfollowing dense medium separation.

FIG. 6 is a graph showing the effect of magnetite type on magnetiterecovery.

FIG. 7 is a graph illustrating the effect of velocity on separation ofcoal feed in dense medium cyclones.

FIG. 8 is a graph illustrating the effect of velocity on the quality ofthe clean coal product from dense medium cycloning.

FIG. 9 is a graph showing a partition curve for classification of a 28mesh by 150 mesh coal feed fraction.

FIG. 10 is a graph showing the effect of magnetite particle sizedistribution on separation of coal feed in dense medium cyclones.

FIG. 11 is a graph showing the effect of magnetite particle sizedistribution on the quality of the clean coal product from dense mediumcycloning.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to a process of beneficiating fineparticle coal through use of a dense medium separation process. Inparticular, the present invention involves a process for beneficiatingparticulate coal particles smaller than about 0.5 mm. The process of thepresent invention results in an exceptionally clean coal product withhigh heating value, low ash and low inorganic sulfur content. Theprocess of the present invention can be used to produce a clean coalproduct which, during combustion, can meet desired emissionspecifications. It has been found that an improved coal product can beproduced by application of one or more of the following methods, andpreferably by application of each of the following methods.

LIBERATION

In one embodiment of the present invention, prior to beneficiation,substantially pure coal and high ash refuse are removed from the rawcoal feed. Coarse coal (coal which is at least 0.5 mm in size) isrelatively easy to clean and satisfactory cleaning processes are knownin the industry. Cleaning of fine coal (coal which is less than about0.5 mm in particle size) is more complex. For example, small particlecoal is much more difficult to separate in dense medium cyclones becausethe small particles have large surface areas and experience high viscousdrag, and because dense media have not traditionally been designed forsuch particles. Accordingly, it is preferable to remove clean coarsecoal prior to beneficiation of coal fines.

A process for treating fine coal which incorporates comminution ofcoarse middlings to liberate coal from non-coal material isadvantageous. Such a need is heightened by recent environmental concernsand regulatory impositions. Coal with high sulfur content will not beacceptable for electrical generation without expensive scrubbing.Comminution, however, is expensive; moreover, cleaning the resultingfines is costly. Comminution should, therefore, be minimized.

The process of the present invention provides an efficient and effectivemeans for removing especially clean coal particles and refuse particlessubstantially barren of coal from a coarse coal feed. By removing coarseclean coal and refuse, only the middling fraction need be comminuted forfurther processing as fine particle coal. Thus, the process has theadvantages of reducing the load on fine particle coal separationequipment, minimizing the cost of comminution and minimizing the amountof fines in the final clean coal product.

In the process of the present invention, raw coal feed is separated bysize into coarse and fine fractions by any suitable method, preferablywith screens. The separation is preferably made at a particle size fromabout 0.25 mm to about 1.0 mm, more preferably from about 0.6 mm toabout 0.4 mm, and most preferably at a size of about 0.5 mm. Theoversize coal is then subjected to dense medium separation, preferablyby dense medium cycloning, at a low specific gravity such that anexceptionally clean coal product is removed as the overflow product.Preferably, the overflow product contains at least about 95 percentcoal. Preferably, the density of separation is no more than about 0.1specific gravity units in excess of the specific gravity of the purecoal being treated. The density of separation refers to the specificgravity for which there is an equal probability that a particle of feedhaving a density corresponding to that specific gravity will report tooverflow or underflow. For example, for a 1.25 specific gravitybituminous coal, the density of separation should be less than about1.35, preferably about 1.30, and for a 1.55 specific gravity anthracitecoal, the density of separation should be less than about 1.65,preferably about 1.60.

The underflow product of this initial separation is preferably subjectedto an additional dense medium separation, preferably by dense mediumcycloning, at a high specific gravity such that non-coal material can beremoved as the underflow product. Preferably, the gravity of separationof this second dense medium separation is at least about 0.5 specificgravity units in excess of the specific gravity of the pure coal, andmore preferably at least about 0.75 specific gravity units in excess ofthe specific gravity of the pure coal. This underflow product issubstantially free of coal and is discarded as refuse. Preferably, theunderflow product contains less than about 25 percent coal morepreferably less than about 15 percent coal. In the alternative, the coalfeed could be subjected to a high gravity separation followed by a lowgravity separation.

The overflow product of the high gravity separation consists ofmiddlings containing a combination of coal and non-coal materials suchas pyrite and other ash-forming minerals. These coal and non-coalmaterials are locked together in the middling product. To liberate thecoal from the non-coal material in the middlings, it is necessary tocrush, grind or otherwise comminute the middlings to a fine particlesize, preferably to less than about 0.5 mm in size. Followingcomminution, the liberated middlings are then processed with the fineparticle coal initially sized away from the coarse fractions.

To assure that no coarse particles pass with the comminuted middlings tobe processed with the fine particle coal, the comminuted middlings maybe recycled to the raw coal feed stream so as to again pass through theinitial sizing step. The undersize from the sizing step, includingcomminuted middlings, is then processed in a separation unitspecifically designed to treat fine particle coal. If desired, prior tothe low and high density separations, the coarse coal can be dividedinto multiple fractions by sizing, and these multiple fractionsindividually subjected to low and high density separations in order toliberate coal from non-coal material. By processing coarse and fine coalseparately, and by comminuting only the middlings, advantages, aspreviously mentioned are realized.

As illustrated in FIG. 2, clean coal and refuse are liberated from a rawcoal feed. A raw coal feed 80 is sized 82 at 0.5 mm. The undersize 84 isrecovered and sent to the dense medium cycloning circuit for smallparticle coal 85. The oversize 86, which is made up of plus 0.5 mmparticles, is subjected to a first dense medium separation 88 at a lowspecific gravity of approximately 1.3. Clean coal 90 is removed as thefloat product of the first dense medium separation 88. The sink product92 from the first dense medium separation 88 is subjected to a seconddense medium separation 94 at a higher specific gravity of approximately2.0. The high gravity sink product 100 is discarded as refuse. The floatproduct 96 of the second dense medium separation 94 is subjected tocomminution 98. The comminuted product 102 is subjected to additionalsizing 82 until the entire coal feed is less than about 0.5 mm in sizeand, therefore, reports to the dense medium cycloning circuit for smallparticle coal 85.

SIZING AND CLASSIFICATION

In another embodiment of the present invention, fine particle coal feedis partitioned into various size fractions prior to cleaning. Cleaningperformance is improved in processes based on specific gravityseparation such as dense medium cycloning when a narrow sizedistribution of coal feed particles is processed. An effective means ofpartitioning fine size coal into extremely fine size fractions allowsfor more efficient separation for fine size coal.

Prior to partitioning, the coal feed is made up of fine size coal. Thefine size coal feed is preferably sized to be smaller than from about0.25 mm to about 1 mm, more preferably smaller than from about 0.4 mm toabout 0.6 mm and most preferably smaller than about 0.5 mm. Preferably,the coal feed is the undersize product of the above-described liberationprocess.

In the process of the present invention, coal is divided into at leastthree size fractions, and preferably into three size fractions, tofacilitate subsequent magnetite recovery and to improve cyclone cleaningperformance. Specifically, the coal is preferably classified at a sizefrom about 0.044 mm to about 0.150 mm, more preferably at a size fromabout 0,085 mm to about 0.125 mm and most preferably at a size of about0.105 mm (150 mesh) by any suitable method such as through use of a fineaperture sieve, preferably a Krebs Varisieve™. Coal smaller than theabove sizes, most preferably smaller than about 0.105 mm, is furtherclassified at a size which allows for the discard of the smallest sizefraction such that an improved clean coal product can be recovered.Classification at a size preferably from about 0.037 mm to about 0.005mm, more preferably from about 0.025 mm to about 0.01 mm and mostpreferably at about 0.015 mm will usually result in the removal of clayslimes which are detrimental if present in sufficient quantities in theclean coal product.

In the past, particles of a certain small size have been removed fromcoal feeds prior to beneficiation because of process limitations such asthe loss of dense medium cyclone cleaning performance with a fineparticle feed. Removal of the smallest size fraction involved theremoval of relatively large size particles as compared to the presentinvention. In the present invention, only an extremely small sizefraction of coal is discarded, for example, slimes less than about 0.015mm in size.

In general, slimes smaller than about 0.015 mm are difficult to separateby dense medium separation and are usually a discard product duringbeneficiation. Removal of these slimes prior to dense medium separationhas the advantage of removing high ash containing particles in arelatively uncomplicated process, together with the advantage ofreducing the load on the dense medium separation equipment. In addition,clay slimes undesirably increase the water retention of cleaned coal,inhibit the recovery of magnetite from the dense medium and, whencombusted, cause slagging in the boiler.

In one embodiment of the present invention, removal of particles smallerthan about 0.015 mm from the fine coal feed is accomplished through theuse of a classifying cyclone. Preferably, the classification circuitconsists of a series of classifying cyclones, and more preferably, theclassifying cyclones are arranged in a countercurrent flow circuit. Itis an advantage of the present process that the removal of extremelysmall size particles by classification in classifying cyclones can beconducted in high throughput capacity, 10" diameter cyclones rather thansmaller diameter cyclones, such as 1" or 2" diameter cyclones,traditionally used to classify extremely small size particles.

When a classifying cyclone is used to classify extremely small size coalparticles, such as particles 0.015 mm in size, most particles largerthan the classification size report to the underflow product and mostsmaller particles distribute in the same proportion as the processwater. When multiple classifying cyclones are used, preferably at leastthree in series, it is also preferable to have the classifying cyclonesconnected in a countercurrent flow arrangement so as to run the flow ofprocess water in a direction opposite to the progression of the raw coalparticles. For example, the underflow from a primary classifying cyclonecontaining coarse coal particles moves to a secondary classifyingcyclone, and the underflow of the secondary classifying cyclonecontaining coarse coal particles moves to a tertiary classifyingcyclone. The overflow of the classifying cyclones containing water andclay slimes is the reverse, namely the overflow from the tertiaryclassifying cyclone flows to the secondary classifying cyclone, theoverflow from the secondary classifying cyclone flows to the primaryclassifying cyclone, and overflow from the primary classifying cycloneis sent to the refuse thickener for discard. In this fashion, thecleanest water is used in the classifying cyclones with the coalcontaining the least clay slimes and the dirtiest water is used todisengage the heaviest clay slimes.

As illustrated in FIG. 3, raw coal 110 reports to a primary VariSieve™112 for sizing at 150 mesh (0.105 mm). The primary underflow 114 reportsto a primary sump 116. The primary overflow 118 reports to a secondarysieve 120 for further sizing at 150 mesh. The secondary underflow 122also reports to the primary sump 116 which now contains minus 150 meshraw coal. The secondary overflow 126 is recovered and sent to a densemedium cycloning circuit for the larger-particle-size coal 128.

The minus 150 mesh raw coal from the primary sump 116 reports to aprimary pump 130, which pumps the primary feed 132 to a primaryclassifying cyclone 134 for further sizing at 15 microns (0.015 mm). Theprimary classifying cyclone 134 overflow 136 which now contains clayslimes is discarded as refuse. The primary classifying cyclone 134underflow 138 reports to a secondary sump 140 serviced by a secondarypump 142. The secondary pump feed 144 reports to a secondary classifyingcyclone 146 for further separation at 15 microns. The secondaryclassifying cyclone 146 overflow 148 splits. Part combines with theprimary VariSieve™overflow 118 for further processing through thesecondary sieve 120. The remainder of the secondary overflow 148 joinsunderflow 122 thence to the primary sump 116. The secondary classifyingcyclone 146 underflow 150 reports to a tertiary sump 152 serviced by atertiary pump 154. Clarified water 156 is added to the tertiary sump152. The tertiary pump feed 158 reports to a tertiary classifyingcyclone 160 for further sizing at 15 microns.

The tertiary classifying cyclone 160 overflow 162 combines with theprimary classifying cyclone 134 underflow 138 for further processing.The tertiary classifying cyclone 160 underflow 164, which is made up of150 mesh by 15 micron size particles, is recovered and sent to the densemedium cycloning circuit for the smaller-particle-size coal 166.

DENSE MEDIUM CYCLONING

A. Cyclone Design Parameters.

In accordance with an embodiment of the present invention, coal lessthan 0.5 mm in size is beneficiated in a dense medium cyclone havingspecific modifications to the design of the cyclone to overcome problemsassociated with cleaning fine size coal particles. A problem withconventional cyclones is that the acceleration of coal and refuseparticles inside the cyclone is too weak to impart an adequate velocityto the fine size particles and, as a result, such particles reportinappropriately to either the underflow or the overflow. The weakness inseparations conducted in conventional cyclones is that smaller coal andrefuse particles have greater fluid resistance or hydraulic drag thanlarger particles. This problem is also encountered in classification ofextremely small particles based on size in classifying cyclones. To beproperly separated, a greater acceleration force needs to be imparted tothe particles to overcome the counter effect of increased drag.

Accordingly, in one embodiment of this invention, an improved process ofbeneficiating fine size coal by dense medium cycloning has beendeveloped. In this embodiment, the cyclone geometry, particularly theinlet area for flow into the cyclone feed chamber, is modified fromconventional cyclones such that it is less than about 0.01 times thesquare of the inside chamber diameter. Preferably, the inlet area forflow is not greater than 0.0096 times the square of the inside diameterof the cyclone feed chamber and is not less than 0.0048 times the squareof the inside diameter of the cyclone feed chamber. Decreasing the inletdiameter, while maintaining the same flowrate as before decreasing theinlet diameter, increases the inlet velocity of the feed which in turnincreases the acceleration force the particles experience inside thecyclone. Increasing the acceleration of the particles in the cycloneimproves separation efficiency. In a preferred embodiment, beneficiationoccurs in a cyclone where the inside diameter of the cyclone feedchamber is about 10 inches and the inlet area for flow is not greaterthan about 0.96 square inches nor less than about 0.48 square inches.

Similar relationships between inlet area for flow and cyclone feedchamber diameter have been found to improve classification performancein cyclones classifying extremely small particles based on size. As withdense medium separation of coal and non-coal materials, the efficiencyof cyclone classification systems used to separate different sizeparticles increases with increased acceleration of the particles, otherparameters being equal.

An advantage of the present invention is that fine particle coal can beeffectively cleaned using a dense medium beneficiation process withoutthe use of froth flotation systems which are difficult to maintain. Ascompared to froth flotation, dense medium cycloning provides advantageswith respect to separation efficiency because not all coals float infroth flotation. In addition, in froth flotation, pyrite tends to floatwith clean coal, thereby contaminating the clean coal product withsulfur contained in the pyrite.

In general, increased particle acceleration improves the separationefficiency of small particle coal, however, increasing particleacceleration without decreasing inlet area also tends to decrease theresidence time of the particles in the cyclone. Excessively shortresidence times decrease the efficiency of separations. In a preferredembodiment of the present invention, the inlet velocity is at least 30ft/sec, more preferably 60 ft/sec and most preferably 90 ft/sec. Thecyclone throughput is selected to provide sufficient residence time toachieve effective separation. Preferably, the throughput isapproximately the industry design standards throughput for a particularcyclone. Once the appropriate inlet velocity and throughput have beenselected, the inlet area can be determined using the relationship:throughput=inlet area ×inlet velocity. As will be appreciated by oneskilled in the art, during actual operation of a cyclone, throughput isoften calculated based on flow pressure measurements, using well knownrelationships.

B. Use of Ultra-fine Magnetite as Dense Media.

In dense medium beneficiation processes, it is advantageous to selectmaterials for use as the dense medium which are easy to remove from coalor refuse after beneficiation. In this manner, the dense mediummaterials can be recycled and reused numerous times. If magneticparticles are selected for use in the dense medium, after beneficiationthese particles can be recovered for reuse utilizing a process whichtakes advantage of their small size and magnetic susceptibilities.Magnetic particles as used herein are those particles capable of beingeffectively separated by magnetic means, and include ferromagnetic orferrimagnetic particles, such as magnetite, ferrosilicon and maghemite.

As noted, dense media used in dense medium separation processes usuallycontain magnetic particles suspended in water. The suspension ofmagnetic particles floats solid particles to be separated similar to adense homogeneous fluid so long as the size of the particles to beseparated are considerably larger than the magnetic particles in thedense medium.

A problem with dense medium cleaning of small size particles has beenthe tendency of clean coal particles to inappropriately report to therefuse underflow of the cyclone. This problem is caused by thedecreasing size difference between the coal particles to be separatedand the particles of the dense medium. As the coal particles becomesmaller relative to the particles of the dense medium, the coalparticles to be separated tend to loose buoyancy and therefore sink. Thedense medium ceases to float particles to be separated in a mannersimilar to a homogeneous dense fluid. As such, ultra-fine size densemedium particles are necessary to effectively separate fine particlesize coal from refuse.

It is an unexpected result of the present invention that dense mediumseparation of fine particle coal is also improved by maintaining thesize distribution of ultra-fine magnetite particles within a narrowrange of sizes. Such narrow distribution of magnetic particle sizes alsoresults in enhanced recovery of the magnetic particles following densemedium separation.

In one embodiment of the present invention, the dense medium is made upof water and a suspension of ultra-fine magnetic particles, preferablymagnetite particles. Preferably, at least about 65 weight percent of themagnetic particles are from about 2 microns to about 10 microns in sizeand no more than about 10 weight percent of such magnetic particles aresmaller than about 2 microns in size. More preferably, at least about 75weight percent of the magnetic particles are from about 2 microns toabout 10 microns in size, no more than about 10 weight percent of suchmagnetic particles are smaller than about 2 microns in size, no morethan about 25 weight percent of the magnetic particles are smaller thanabout 3 microns in size and at least about 10 weight percent of themagnetic particles are larger than about 7 microns in size.

C. Production of Ultra-fine Magnetite.

In one embodiment of the present invention, an ultra-fine magnetite isproduced for use in connection with dense medium beneficiation of coalfeed with a particles sized at less than about 0.5 mm. Commerciallyprepared magnetite is too large to effectively separate fine particlesize coal and refuse. To effectively separate coal down to about 0.015mm, magnetite particles are preferably less than about 0,010 mm in size,and preferably with at least about 50 percent of the particles less thanabout 0,005 mm in size. It has been found that a superior magnetite,with most particles smaller than about 0.010 mm and preferably at leastabout 90 percent below about 0.010 mm, can be produced by the process ofthis invention.

Two processes for producing ultra-fine magnetite are:

1) spray-roasting a solution of ferrous chloride in air to producehematite by pyrohydrolysis, followed by chemically reducing hematite tomagnetite. The chemical reactions for this process are:

(a) production of hematite by pyrohydrolysis of iron chloride:

    FeCl.sub.2 +H.sub.2 O+1/4O.sub.2 →1/2Fe.sub.2 O.sub.3 +2HCl

(b) Reduction of hematite to magnetite by the use of either hydrogen orcarbon monoxide, or both:

    3Fe.sub.2 O.sub.3 +H.sub.2 →2Fe.sub.3 O.sub.4 +H.sub.2 O

    3Fe.sub.2 O.sub.3 +CO→2Fe.sub.3 O.sub.4 +CO.sub.2

2) spray-roasting a solution of ferrous chloride in restricted airdirectly to magnetite by pyrohydrolysis. The chemical reaction for thisprocess is:

    FeCl.sub.2 +H.sub.2 O+1/6 O.sub.2 →1/3 Fe.sub.3 O.sub.4 +2HCl

It is preferable that the ratios of concentrations of product gas toreactant gas are limited such that the reduction of hematite tomagnetite does not proceed beyond magnetite to ferrous oxide, FeO, oreven metallic iron.

Preferably, the magnetite particles are produced by the reduction ofhematite or the direct pyrohydrolysis to magnetite under reducingconditions, including residence time and temperature, designed tocontrol crystal growth of the magnetite which is produced. The resultingmagnetite particles preferably have the narrow size distribution setforth above in Section B.

In one embodiment of the invention, ferrous chloride is spray-roasted inair to form hematite by pyrohydrolysis. Hematite produced byspray-roasting is then reduced to magnetite at a temperature and for aperiod of time sufficient to limit magnetite crystal growth. Preferably,reduction of the magnetite occurs at a temperature of from about 900° C.to about 1000° C., and more preferably from about 980° C. to about 1000°C., for a period of time to produce magnetite crystal growth whichresults in a narrow size distribution of magnetite particles withoutdecreasing the separation efficiency of a dense medium separationprocess employing the magnetite. Preferred size distributions are setforth hereinbefore in Section B.

Any suitable reactor for the reduction of hematite to magnetite can beused, such as rotary kiln reactors. In a preferred embodiment, thespray-roasted hematite is pelletized prior to reduction to magnetite.Pelletization avoids the problem of the hematite being blown out of thereactor prior to reduction to magnetite.

Preferably, hematite is reduced to magnetite with carbon monoxide andhydrogen with the flow of reducing gases countercurrent to the flow ofhematite pellets which are fed into the reactor opposite the burnerflame, and which are heated to greater temperatures as the pellets movethrough the reactor chamber, attaining maximum temperature near theproduct discharge. Reducing conditions are preferably maintainedthroughout the reactor, such as by injection of additional reducing gasinto the reactor near the product discharge end.

In a preferred embodiment, the magnetite pellets are broken up bycomminution or attrition scrubbing to provide magnetite particles havinga size on the order of the natural grain size of the magnetite afterreduction. As will be appreciated, the preferred size distributions forthe magnetite particles set forth in Section B above are the sizes ofthe particles as broken up. It is also preferable to remove any solublechlorides, especially alkaline earth chlorides which are not volatile,from the magnetite by, for example, countercurrent washing or throughwashing during attrition scrubbing.

Magnetite produced by the process of this invention results in severaladvantages when used in connection with the beneficiation of smallparticle coal. Specifically, magnetite produced by this process can bemore easily recovered magnetically, thereby reducing the cost ofbeneficiation by reducing magnetite loss and the amount of magneticseparation equipment required. Also as a result of the ease ofrecovering the magnetite, operating costs are reduced. In addition, themagnetite particles produced by the process of this invention have anarrow size distribution (the size of most particles is close to themedian particle size and relatively few particles are substantiallysmaller or larger than the mean particle size). This narrow sizedistribution results in improved separation efficiency in the densemedium beneficiation process.

RECOVERY OF MAGNETIC PARTICLES

In another embodiment of the present invention, magnetic particles,preferably magnetite, are recovered separately from the clean coal andrefuse fractions after beneficiation. Preferably, different approachesfor magnetic particle recovery are applied to cleaned fractions composedof particles of different size ranges.

A drain and rinse approach followed by magnetic separation is applied torecover magnetic particles from the rinse water from a fractioncontaining larger-size coal or refuse particles, such as a fractioncontaining particles smaller than about 0.4 mm to about 0.6 mm, andlarger than about 0.085 mm to about 0.125 mm. The majority of themagnetite is recovered from this larger-particle-size fraction as itflows across sieves, preferably a series of at least two sieves and mostpreferably a series of three sieves. Draining is followed by rinsingwith clean process water, preferably employing a countercurrent systemwhere the flow of rinse water is countercurrent to the flow of coal orrefuse particles. Preferably, additional magnetite and moisture areremoved from the coal on a final vibrating screen followed by recoveryof the magnetic material from the dilute or rinsed stream. It ispreferable that the coal and refuse fractions from which the medium hasbeen drained are repulped through the addition of water and then sent tothe next sieve where the bulk of the water passes through the sievecarrying the bulk of the magnetite. The drained medium from the firstsieve containing magnetite and water can be recycled directly to a densesump for use as the dense medium in dense medium cyclones.

Recovery by the drain and rinse approach of magnetic particles less thanabout 0.01 mm in size from a fraction of coal or refuse containingsmaller-sized-coal or refuse particles, such as a fraction containingparticles smaller than about 0.085 mm to about 0.125 mm and larger thanabout 0.01 mm to 0.02 mm, is more difficult because sieves or screenswhich produce a size separation for industrial quantities in the 0.01 mmto 0.07 mm size range are generally not available.

According to an embodiment of this invention, magnetic particles,preferably magnetite particles, are recovered from asmaller-particle-size fraction of refuse and coal in a series ofmagnetic separators specially adapted for the recovery of ultra-finemagnetic particles. This recovery scheme is identified as arougher-cleaner-scavenger circuit. The rougher is one or more wet drummagnetic separators, preferably three wet drum separators in series,having standard strength magnets such as barium ferrite magnets.

The coal or refuse stream is initially run through the magneticseparators comprising the rougher step. Preferably, the coal or refusefeed to the rougher step is diluted with a recycle stream ofnon-magnetic particles from the cleaner separator. Dilution improvesmagnetite recovery from the smaller-particle-size fractions of coal orrefuse.

Non-magnetic effluent water from the magnetic particle recovery processfor the larger-particle size fractions of coal or refuse still containssmall quantities of both coal and magnetic particles, which can beremoved by combining said effluent with the smaller-particle-sizeprocess stream prior to magnetic recovery, preferably prior to densemedia separation of the smaller-particle-size stream.

The magnetic concentrate separated out by the magnets of the rougherstage is diluted with water and then sent to a cleaner stage which ismade up of wet drum magnetic separators which contain standard strengthmagnets such as barium ferrite. The magnetic concentrate separated outin the cleaner stage is recycled for makeup of dense media to be used indense medium separation. The non-magnetic effluent exiting the cleanerstage can be recycled as dilution water to dilute feed to the rougherunit.

Finally, the non-magnetic effluent from the rougher and cleaner stagesincluding the coal or refuse particles, and still containing a smallamount of magnetic particles, is sent to the scavenger stage. Thescavenger stage includes wet drum magnetic separators containing magnetsstronger than those used in the rougher and cleaner stages. Preferably,the magnets used in the scavenger contain rare earth magnets.Preferably, the drum of the scavenger separator has been positioned tobring the magnetic particles closer to the magnet by narrowing the gapin the magnetic separator.

The magnetite recovery process of this invention is advantageous in thatthe least expensive method of recovery, namely drain and rinse, isapplied where possible, while the more expensiverougher-cleaner-scavenger circuit is used to recover only the finestmagnetite.

FIG. 4 depicts a magnetite recovery circuit following dense mediumseparation of larger-particle-size coal or refuse fractions. Forexample, a fraction of 0.5 mm by 150 mesh (0.105 mm) coal or refusecould be processed by this circuit. The circuit includes a series ofrinse sieves with process water flowing between the sieves in flowcountercurrent to the flow of coal or refuse. Referring to FIG. 4, thecoal or refuse fraction from dense medium cycloning 200 feeds to thefirst sieve 202 where dense media drains from the coal or refuse. Thedrained media 204 returns to the dense media sump for reuse. Theoversize coal or refuse 206 then goes to a sump 208, where the coal orrefuse is repulped with water and the resulting slurry 210 is pumped tothe first rinse sieve 212 where water and magnetite particles 214 drainfrom the coal or refuse. A spray rinse to remove additional magnetitecould also be incorporated. The drained water and magnetite 214 go to awet drum magnetic separator 216 where a magnetite concentrate 218 isrecovered and sent to the overdense media sump for reuse. Cleanedeffluent 250, which still contains a small quantity of magnetite, fromthe magnetic separator can be mixed with a smaller-particle-size coalfeed fraction, such as a 150 mesh (0.105 mm) by 15 micron (0.015 mm)fraction for example, preferably reports to a thickener (not shown) andthen dense medium separation of that fraction. Remaining magnetite istherefore recovered from the smaller-particle-size fraction.

From the first rinse sieve 212, the oversize coal or refuse 222 is thenrepulped with water in sump 224 and the resulting slurry ..226 is pumpedto the second rinse sieve 228, where water and magnetite 230 are drainedfrom the coal or refuse. The drained water and magnetite 230 are usedfor repulping in the first sump 208.

From the second rinse sieve 228, the oversize coal or refuse 232 goes tosump 234 and is repulped with clean process water 236 and the resultingslurry 238 is then pumped to a third rinse sieve 240 where magnetite andwater 242 drain and then go to sump 224 as repulping liquid.

From the third rinse sieve 240, the oversize coal or refuse 243 goes toa dewatering screen 244 where clean process water 236 is sprayed ontothe screen to dislodge and rinse away remaining magnetite. The rinsewater containing magnetite 246 is then used as repulping liquid in thethird sump 234. The coal or refuse product 248 is removed as oversizefrom the dewatering screen 244.

FIG. 5 illustrates a magnetite recovery circuit which follows densemedium separation of smaller-particle-size coal or refuse fractions. Forexample, a fraction of 150 mesh (0.105 mm)×15 micron (0.015 mm) coal orrefuse could be processed in this circuit. Referring to FIG. 5,smaller-particle-size coal or refuse 300 from a dense medium cyclone isdiluted with dilution water 302. The dilution water 302 can originatefrom a number of sources, for example, from non-magnetic effluent 306from the cleaner stage 308 of the present process or fresh or recyclewater (not shown).

The diluted dense media cyclone overflow (coal) or underflow (refuse)310 enters the rougher stage 312 and passes through three wet drumseparators 314, 316, 318. Preferably the wet drum separators havestandard strength magnets, such as barium ferrite magnets. The magnetics320 are diluted with dilution water 322 and sent to cleaner stage 308for processing by a wet drum separator 324. Preferably, the wet drumseparator 324 contains a standard strength magnet. The magneticconcentrate 326 is sent to a magnetic concentrate overdense media sumpand pump where it can be recycled for use in subsequent dense mediumseparations. As previously stated, the non-magnetic effluent 306 exitingthe cleaner stage 308 can be employed as dilution water 302 for thedense media cyclone overflow (coal) or underflow (refuse) 300. Theexcess not required for dilution 332 can be sent to a scavenger stage334 along with the non-magnetic effluent 330 from the rougher stage 312.In the scavenger stage 334 a wet drum separator 338 employing a strongmagnet, such as a rare earth magnet, is employed to separate theremaining magnetics 340 from the clean coal or refuse 342. The magnetics340 from the scavenger stage 334 can be recycled 324 for use in densemedium separations. The clean coal or refuse 342 can be sent to athickener 344.

DEWATERING AND AGGLOMERATION

In another embodiment of the present invention, after recovery ofmagnetite from the clean coal product, the coal is dewatered usingconventional methods such as centrifuge or vacuum filtration. Dewateringto reduce coal moisture content is advantageous prior to combustion ofcoal. In a preferred embodiment, paper fibers, preferably newsprintfibers, are added to a coal and water slurry prior to dewatering cleancoal fractions. Preferably, the size of coal particles in the fractionis less than from about 0.085 to 0,125 mm and greater than from about0.010 to 0.020 mm. The addition of paper fibers results in severalimprovements to the dewatering process, particularly including: (1)increased moisture reduction during dewatering, (2) improved strength ofcoal pellets produced by agglomerating with binder, or equal strengthcoal pellets with less binder, (3) improved ignition of the coal, (4)increased BTU's, and (5) an environmental advantage from a beneficialuse of paper waste.

In another embodiment of the present invention, the clean coal product,particularly a smaller-particle-size fraction, is subjected toagglomeration using suitable agglomeration techniques.

FIGS. 1A and 1B depict process flow for one embodiment of the invention.Raw coal feed 1 sized in the first sizing unit 2, by sieves, screens, orother suitable methods. Oversize particles 4, those over 0.5 mm in size,for example, are then sent to a high gravity separation unit 5 whichunit involves density separation by jigs, dense medium or other suitablemethods. Sink from the high gravity separation unit 5 is discarded asrefuse 6. Float 7 then goes to a low density separation unit 8 wheredense medium separation occurs. Float exits as a clean coal product 9,ultra-fine magnetite 62 is added as needed. Sink from the low gravityseparation unit 8 constitutes middlings 10 which are sent to acomminution unit 11 where the middlings are crushed, ground, orotherwise comminuted, and the comminuted middlings 71 are then combinedwith the raw coal feed 1 for further processing.

Magnetite is recovered by any suitable method following low gravityseparation and recovered magnetite and water 12 are sent to a thickener13 where water 14 is removed. The thickened magnetite with some water 15then goes to the dense media sump 16.

Undersize 3 from the first sizing unit 2 is sent to a second sizing unit18 where particles are sized by sieve, screen, or other known methods.Undersize 20, for example minus 150 mesh (0.0105 mm) particles then goesto a classifying cyclone circuit 21 designed to classify at anultra-fine particle size, at 15 microns (0.015 mm) for example. Theslimes exiting with the overflow 22 are discarded as refuse. Processwater 23 is added to the classifying cyclone circuit 21, which isoperated in countercurrent flow. Underflow 24, is sent to the thickener13 where the underflow is thickened along with magnetite from the lowdensity separation unit 8. The thickened slurry 15 then goes to thedense media sump 16 where ultra-fine magnetite 61 is added as needed.Slurry 17 from the dense sump 16 goes to dense medium cyclones 25 fordense medium separation. Overflow 26 containing clean coal then goes toa magnetic separation unit 58 where magnetite is removed in wet drummagnetic separators in a rougher-cleaner-scavenger arrangement, with arare earth magnet incorporated into the scavenger separator to enhancemagnetite recovery. The clean coal 64 then goes to a dewatering unit 56where paper fibers 66 are added to the coal prior to dewatering bycentrifuge. The dewatered coal 67 then goes to an agglomeration unit 68where the coal is pelletized with the aid of a binder 69 if needed.Clean coal pellets 70 exit as a final product. Underflow 27 from thedense medium cyclones 25 containing refuse goes to a magnetic separationunit 28 which operates the same as the magnetic separation unit 58previously described for the overflow. Magnetite-free refuse 29 exitsthe magnetic separation unit 28 to be discarded.

Concentrated magnetite 30 and 59 from the magnetic separation units 28and 58 combine 60 and go to the over-dense media sump 46. Over-densemedia 47 from the over-dense media sump goes to the dense medium sump31. Over-dense media 63 is also sent to the low gravity separation unit8 and dense media sump 16 as needed.

Oversize 19 form the second sizing unit 18, for example 0.5 mm by 150mesh (0.105 mm) particles go to dense medium sump 31, where processwater 72 and over-dense medium 47 are added to establish the properslurry density. Magnetite used in the dense medium sump 31 is ofultra-fine particle size, with over 60 percent of the particles between10 microns and 2 microns in size.

Slurry 32 from the dense medium sump 31 goes to dense medium cyclones 33for dense medium separation. Overflow 34 is first drained 36 over asieve and the oversize clean coal 37 is then rinsed 39 over screens withprocess water 40 added. Clean coal product 41 exits from the rinsingstage 39.

Rinse water containing magnetite particles 42 is then processed in amagnetic separation unit 43 containing one or more wet drum magneticseparators, where magnetite is removed from the water.

Magnetite from underflow 35 from dense media cyclones 33 is recoveredsimilarly to magnetite recovery from overflow 34, just described.Magnetite and water are drained 49 with drain liquid 55 being combinedwith drain liquid from the overflow 38 and then being sent to the densemedium sump 31. The oversize refuse 50 is rinsed 51 with process water52. Oversize refuse 53 then exits the rinsing circuit, to be discarded.Rinse water containing magnetite 54 goes to a magnetic separation unit56 containing one or more wet drum magnetic separators. Water 58 and 45exiting the magnetic separation units, which still contains smallamounts of magnetite, combine 48 and go to the thickener 13, from whichthe magnetite continues in the process flow as previously described andis ultimately recovered.

EXAMPLE 1 Magnetite Production

Magnetite was produced by the reduction of hematite in kiln reactors attwo different temperatures. The hematite feed had been previouslyproduced by spray roasting of iron chloride in a pyrohydrolysisreaction. The hematite was fed into one end of the kiln and magnetiteproduct was recovered from the opposite end of the kiln. The hematitewas heated as it moved through the kiln and reached maximum temperaturenear the discharge end. Either hydrogen gas or natural gas was injectedinto the kiln to insure a reducing environment throughout the kiln. Themagnetite product was broken down to the natural grain size by crushingand attrition scrubbing as necessary. Particle size was then measured.

Magnetite was first produced with a maximum temperature in the reactorof approximately 750° C. Next, magnetite was produced with a maximumtemperature in the reactor of approximately 1000° C. Table 1 shows asize analysis comparison of the two magnetite products. The magnetiteproduced at a temperature of approximately 750° C. is denoted M1 andmagnetite produced at a temperature of approximately 1000° C. is denotedM2.

Surprisingly, M2 contains a much narrower distribution of particlesizes, with approximately 80 weight percent of magnetite particles fromabout 2 microns to about 10 microns in size. The M1 magnetite has a muchwider particle size distribution and only approximately 50 percent ofthe magnetite particles are between 2 and 10 microns in size. Althoughthe exact reason for this difference in size distribution is not fullyunderstood, and while not wishing to be bound by any theory, it is feltthat limited recrystallization of magnetite in the 1000° C. reaction wassufficient to narrow grain size distribution, but that recrystallizationdid not proceed to such an extent that excessive particle growthoccurred.

The relative absence of particles larger than 10 microns and particlessmaller than 2 microns in M2 magnetite is advantageous for dense mediumcyclone separation as shown in Example 4. The narrow size distributionof M2 magnetite is also advantageous for enhanced recovery of magnetiteby magnetic separation following dense medium separation. FIG. 6 shows agraph of magnetite response to differing magnetic field strengths. Thegraph shows the amount of magnetite recovered in a Davis Tube separatoras a function of magnetic intensity expressed as the current flowingthrough the coil of the electromagnet. M2 magnetite shows greaterresponse to lower intensity magnetic fields and is therefor easier torecover in magnetic separators following dense medium separation.

                  TABLE 1                                                         ______________________________________                                        Magnetite Particle Size Distribution                                                    WEIGHT PERCENT  SMALLER THAN                                        SIZE      M1              M2                                                  ______________________________________                                        44.0 microns                                                                            99.9%           98.0                                                31.1      98.9            97.2                                                22.0      93.7            95.2                                                15.6      86.2            92.1                                                11.0      75.4            91.4                                                 7.78     67.5            85.9                                                 5.50     59.8            70.0                                                 3.89     51.6            41.1                                                 2.75     38.2            19.8                                                 1.94     21.8            8.2                                                  1.38     11.5            3.2                                                  0.97     3.7             0.5                                                 ______________________________________                                    

EXAMPLE 2 Classifying Cyclone Performance

Samples of minus 150 mesh (0.105 mm) Sewickley Seam coal were classifiedin a cyclone with varying inlet areas. The tests were in a 10" diametercyclone. Inlet pressure was varied to maintain approximately equal feedrates for each test. Feed rates were within the normal range forindustry design standards for the particular cyclone design. Thus theeffect of increasing acceleration on particle separation could beevaluated as a function of velocity, without increasing volumetric flowrate beyond the industry design standards for the particular cyclone.

The inlet areas tested were 3.1 square inches, 0.96 square inches, and0.48 square inches, with corresponding velocities of about 16 feet persecond, 56 feet per second, and 104 feet per second, respectively. Table2 shows the particle size at which 50 percent of the particles of thatsize report to overflow and 50 percent to underflow for each test. Thesetest results show that classification occurs at a smaller size as inletvelocity is increased at a constant volumetric feed rate.

                  TABLE 2                                                         ______________________________________                                        Classifying Cyclone Performance                                                                             CLASSIFICATION                                  CYCLONE  INLET     INLET      SIZE (50%                                       DIAMETER AREA      VELOCITY   CUT POINT)                                      ______________________________________                                        10"      3.10"     16 ft/sec. 31.8 microns                                    10"      0.96"     56 ft/sec. 11.3 microns                                    10"      0.48"     104 ft/sec.                                                                               5.5 microns                                    ______________________________________                                    

EXAMPLE 3 Dense Media Cyclone Performance

Three tests were run to evaluate the effect of varying the inlet area,and consequently inlet velocity, on dense media cyclone separation atapproximately constant volumetric flow rate. Sewickley Seam coal sizedat 150 mesh (0.105 mm) by 15 microns (0.015 mm) was separated in 10"diameter cyclones. Inlet pressure was varied to maintain approximatelyequal inlet feed flow rates, within industry design standards for theparticular cyclone, for all tests so that the effect of acceleration onseparation could be evaluated as a function of inlet velocity. Inletareas tested were 3.1 square inches, 0.96 square inches and 0.48 squareinches, with corresponding velocities of 20.6 feet per second, 66.6 feetper second, and 133.2 feet per second, respectively. Dense mediacontained M2 magnetite, as shown in Table 1, for all tests.

FIGS. 7 and 8 summarize results of the tests. FIG. 7 shows that theyield of clean coal product increases significantly with increasinginlet velocity into the cyclone. FIG. 7 also shows that a greaterpercentage of heating volume in the coal feed is recovered in the cleancoal product at higher velocities. FIG. 8 shows that the clean coalproduct was of high quality in all tests. Thus, increasing the inletvelocity while holding the volumetric feed rate constant resulted in asubstantial increase in the yield of clean coal without compromising thequality of the clean coal product.

Another test was run with an inlet area of 0.48 square inches and aninlet velocity of 133.2 feet per second on a 28 mesh (0.596 mm) by 150mesh (0.105 mm) coal feed. FIG. 9 shows the partition curve for thistest and a probable error of 0.032. High inlet velocities to cyclones atindustry design standards for volumetric feed rates results in goodseparation of coal and non-coal material from fine coal feed.

EXAMPLE 4 Effect Of Magnetite Type on Dense Media Separation

Two tests were run to evaluate the separation performance using twodifferent magnetite types, M1 and M2. Particle size distributions of M1and M2 magnetites are shown in Table 1. Both tests were run usingSewickley Seam coal sized at 150 mesh (0.105 mm) by 15 microns (0.015mm). Both tests were conducted in 10" cyclones with approximately equalfeed rates. FIGS. 10 and 11 summarize the results of the tests anddemonstrate improved separation efficiency with the M2 magnetite. FIG.10 shows the result that M2 magnetite improves the yield of clean coalproduct and the BTU recovery. FIG. 11 shows, surprisingly, that use ofM2 magnetite also improved the quality of the clean coal product byreducing the ash and sulfur content and by increasing the BTU content ofthe clean coal product. M2 magnetite, even though having a largeraverage particle size than M1 showed improved separation efficiency.Thus, the size distribution of magnetite particles, and not justparticle size, affects both coal separation efficiency and magnetiterecovery efficiency.

Although the preferred embodiment has been described by way ofillustration and example, as known to those skilled in the art, a numberof variations and modifications of the invention can be practiced withinthe scope of the present invention as limited only by the appendedclaims.

What is claimed is:
 1. A process for beneficiating coal comprising:(a)providing a coal feed comprising particulate solids; (b) dividing thecoal feed into at least three fractions based on particle size; (c)discarding as refuse a first of said fractions from step (b), whichfirst fraction contains the smallest-size particles; (d) separatelyprocessing the remaining fractions from step (b) in dense mediumseparation units, with dense medium comprising liquid and magneticparticles, to separate each fraction into a clean coal overflow and arefuse underflow; (e) separately recovering magnetic particles from eachof the overflow and underflow following dense medium separation of asecond fraction from (b), which contains larger-size particles processedin step (d), by draining and then rinsing with water on a screen orsieve followed by removal of dense medium magnetic particles from therinse water by magnetic separation; and (f) separately recoveringmagnetic particles from each of the overflow and underflow followingdense medium separation of a third fraction from (b), which containssmaller-size particles processed in step (d), by magnetic separation;wherein said first fraction from step (b), which is discarded in step(c), comprises the overflow from a classifying cyclone comprising aninlet orifice and a feed chamber, and wherein the average velocity ofthe coal feed through the inlet orifice by which the feed enters thecyclone feed chamber is at least 60 feet per second.
 2. The process ofclaim 1, wherein the average velocity of the coal feed through the inletorifice of said classifying cyclone by which the coal feed enters thecyclone feed chamber is at least 90 feet per second.
 3. A process forbeneficiating coal, comprising:(a) providing a coal feed comprisingparticulate solids; (b) dividing the coal feed into at least threefractions based on particle size; (c) discarding as refuse a first ofsaid fractions from step (b), which first fraction contains thesmallest-size particles; (d) separately processing the remainingfractions from step (b) in dense medium separation units, with densemedium comprising liquid and magnetic particles, to separate eachfraction into a clean coal overflow and a refuse underflow; (e)separately recovering magnetic particles from each of the overflow andunderflow following dense medium separation of a second fraction from(b), which contains larger-size particles processed in step (d), bydraining and then rinsing with water on a screen or sieve followed byremoval of dense medium magnetic particles from the rinse water bymagnetic separation; and (f) separately recovering magnetic particlesfrom each of the overflow and underflow following dense mediumseparation of a third fraction from (b), which contains smaller-sizeparticles processed in step (d), by magnetic separation; wherein thedense medium separation units in step (d) comprise dense medium cycloneseach comprising an inlet orifice and a cyclone feed chamber, and whereinthe average velocity of the feed through the inlet orifice to enter saidcyclone feed chamber is at least 60 feet per second.
 4. The process ofclaim 3, wherein the average velocity of the feed through the inletorifice to enter said cyclone feed chamber is at least 90 feet persecond.
 5. A process for beneficiating coal, comprising:(a) providing acoal feed comprising particulate solids; (b) dividing the coal feed intoat least three fractions based on particle size; (c) discarding asrefuse a first of said fractions from step (b), which first fractioncontains the smallest-size particles; (d) separately processing theremaining fractions from step (b) in dense medium separation units, withdense medium comprising liquid and magnetic particles, to separate eachfraction into a clean coal overflow and a refuse underflow; (e)separately recovering magnetic particles from each of the overflow andunderflow following dense medium separation of a second fraction from(b), which contains larger-size particles processed in step (d), bydraining and then rinsing with water on a screen or sieve followed byremoval of dense medium magnetic particles from the rinse water bymagnetic separation; and (f) separately recovering magnetic particlesfrom each of the overflow and underflow following dense mediumseparation of a third fraction from (b), which contains smaller-sizeparticles processed in step (d), by magnetic separation; wherein thedense medium in step (d) comprises water and magnetite particles andwherein said magnetite particles are produced from reduction ofhematite, wherein said reduction takes place under such conditions ofresidence time and temperature that crystal growth of magnetite producedis limited such that magnetite particles produced are of a size suchthat at least about 60 weight percent of such magnetite particles arefrom about 2 microns to about 10 microns in size.
 6. The process ofclaim 5, wherein said magnetite particles produced are of a size suchthat at least about 75 weight percent of such magnetite particles arefrom about 2 microns to about 10 microns in size.
 7. A process forbeneficiating coal, comprising:(a) providing a coal feed comprisingparticulate solids; (b) dividing the coal feed into at least threefractions based on particle size; (c) discarding as refuse a first ofsaid fractions from step (b), which first fraction contains thesmallest-size particles; (d) separately processing the remainingfractions from step (b) in dense medium separation units, with densemedium comprising liquid and magnetic particles, to separate eachfraction into a clean coal overflow and a refuse underflow; (e)separately recovering magnetic particles from each of the overflow andunderflow following dense medium separation of a second fraction from(b), which contains larger-size particles processed in step (d), bydraining and then rinsing with water on a screen or sieve followed byremoval of dense medium magnetic particles from the rinse water bymagnetic separation; and (f) separately recovering magnetic particlesfrom each of the overflow and underflow following dense mediumseparation of a third fraction from (b), which contains smaller-sizeparticles processed in step (d), by magnetic separation; wherein saidcoal feed in step (a) is comprised of a middling fraction, said middlingfraction being the mid-density fraction from a separation of aparticulate stream into three fractions based on density, said middlingfraction having been comminuted to a maximum particle size of from about0.6 mm to about 0.4 mm.
 8. A process for separating solid particles,comprising:separating solid particles by dense medium separation withdense medium comprising water and magnetite particles, wherein at leastabout 60 weight percent of said magnetite particles are from about 2microns to about 10 microns in size, wherein said magnetite particlesare produced from reduction of hematite, wherein said reduction takesplace under such conditions of residence time and temperature thatcrystal growth of magnetite produced is limited such that magnetiteparticles produced are of a size such that at least about 60 weightpercent of such magnetite particles are from about 2 microns to about 10microns in size.
 9. The process of claim 8, wherein said magnetiteparticles produced are of a size such that at least about 75 weightpercent of such magnetite particles are from about 2 microns to about 10microns in size.
 10. A process for separating solid particles,comprising:separating solid particles by dense medium separation withdense medium comprising water and magnetite particles, wherein at leastabout 60 weight percent of said magnetite particles are from about 2microns to about 10 microns in size, wherein said magnetite particlesare produced by spray roasting an aqueous solution of iron chlorideunder such conditions of residence time and temperature that crystalgrowth of magnetite produced is limited such that magnetite particlesproduced are of a size such that at least about 60 weight percent ofsaid magnetite particles are from about 2 microns to about 10 microns insize.
 11. The process of claim 10, wherein said magnetite particlesproduced are of a size such that at least about 75 weight percent ofsaid magnetite particles are from about 2 microns to about 10 microns insize.
 12. A process for classifying solid particles according toparticle settling velocities comprising:(a) forming a slurry comprisingsolid particles to be classified and water; (b) feeding the slurry to aclassifying cyclone having an inlet orifice and a cyclone feed chamber,wherein the average inlet velocity through the inlet orifice to thecyclone feed chamber is at least about 90 feet per second; and (c)recovering overflow and underflow streams from said classifying cyclone,said streams comprising solid particles classified by size.